Pretty Little Liars: Go Behind the Scenes With the Cast!

The storylines may be getting darker on ABC Family's Pretty Little Liars, but the mood on set is lighter than ever. "Our show has such heavy material, everyone can feel on edge," Keegan Allen, who plays Toby Cavanaugh, says in the Pretty Little Liars Collector's Edition of Us Weekly (available now). "Then someone sits on a whoopee cushion, and we'll all of a sudden feel better!"

The girls -- Shay Mitchell, Troian Bellisario, Lucy Hale and Ashley Benson -- usually arrive on set at 5:30 a.m. "We're in hair and makeup for an hour and a half!" says Benson, 23. Night shoots, which can go until 3 a.m., "are like slumber parties on the set," according to Mitchell.

In between takes, the costars often interact with their fans via social media. "Ashley and I are always laughing and taking 'selfie' pictures," says Benson's onscreen beau, Tyler Blackburn. When they're filming major episodes, however, executive producer I. Marlene King says she enforces strict rules to prevent spoiler leaks: "No tweeting! No Instagram! And shred your script!"

Even the cast is kept in the dark about the show's many twists and turns. "We get the script about two days before we start filming an episode," says Bellisario, 27. "But usually one of us will try to steal it beforehand. We can't wait to find out what happens next!"